LEDs in recent years have experienced a rapid improvement in their performance, and white LEDs, which are essential for illumination, have been developed. Also in terms of brightness, LEDs have become capable of emitting light that is bright enough to be used for illumination lamps.
However, since forward voltages of LEDs, constituting an illumination lamp which is made up of aggregate of the LEDs, vary, driving currents of the LEDs vary. As a result, luminosities of the LEDs also vary. In addition, in a case where the luminosities of the LEDs vary, partial unevenness occurs in brightness on a light-emitting surface of the illumination lamp. This reduces quality of the illumination lamp.
In view of the circumstances, a configuration of a light-emitting diode lighting system (illumination device) has been conventionally known, in which configuration a constant current circuit is provided for each LED or for each series circuit constituted by a plurality of (approximately six) LEDs connected in series. The constant current circuit controls a current passing through its corresponding LED(s) to be constant, thereby reducing variations in luminosities of LEDs.
FIG. 4 is a circuit diagram showing an example of a circuit configuration of a conventional light-emitting diode lighting system.
In a lighting circuit 1001, a power source Vdd1 and a constant current circuit 1003, which are for turning on an LED array 1011 constituted by six LEDs 1 connected in series, are connected. Each of the LEDs 1 has a forward voltage drop (Vf) of 3.6 V (typ.: typical) and emits white light. The constant current circuit 1003 controls a transistor 1006 so that a current, which is determined by a voltage value of a reference voltage (Vref) and a resistance value of a resistor 1004, is stably supplied to the LED array connected to the constant current circuit 1003. Patent Literature 1 discloses how such a lighting circuit for LEDs operates. In this configuration, the LED array 1011 have a voltage drop of 21.6 V, which is the sum total of the Vfs of the LEDs 1. Therefore, the voltage value of the Vdd1 is set at 30 V in consideration of variations in the Vfs of the LEDs 1 and changes in power supply.
In a lighting circuit 1002, a power source Vdd2 and a constant current circuit 1003, which are for turning on an LED array 1012 constituted by six LEDs 2 connected in series, are connected. Each of the LEDs 2 has a forward voltage drop (Vf) of 2.1 V (typ.) and emits warm color light. In this configuration, the LED array 1012 has a voltage drop of 12.6 V, which is the sum total of the Vfs of the LEDs 2. Therefore, the voltage value of the Vdd2 is set at 20 V in consideration of variations in the Vfs of the LEDs 2 and changes in power supply.
However, the conventional circuit configuration as shown in FIG. 4 has the following problem. In order to turn on a plurality of LEDs which emit light of different colors and control the colors of the light, many power sources are necessary and this leads to cost increase.
Note, here, that it is possible to solve the problem of high costs by modifying the configuration such that anodes of the LED arrays 1011 and 1012 are connected to an identical point and the point is connected to a common power source.
However, in a case where the anodes are connected to an identical point, it is necessary that the power supply voltage of each of the power sources Vdd1 and Vdd2 be 30 V. In such a case, a voltage applied to the constant current circuit 1003 of the lighting circuit 1002 simply increases by 10 V. In view of this, for the purpose of preventing a current which passes through a resistor 1004 from changing, the constant current circuit 1003 changes an output voltage of an operational amplifier which serves as a comparator 1005 so as to change a gate voltage of a transistor 1006, thereby increasing on-resistance of the transistor 1006. As a result, more current is consumed by the transistor 1006, and more heat is generated. That is, according to the system in which the anodes of LEDs having different Vfs are connected to an identical point like above, a problem of power waste occurs. In addition, the above system also causes a problem that, since LEDs are heat-sensitive, it is necessary to take additional measures to dissipate generated heat to prevent deterioration caused by high temperature. The same applies to a case where cathodes are connected to an identical point.
Note that an example of the configuration of FIG. 4 is shown in FIG. 15 of Patent Literature 1.
In order to solve the above problem of heat generation, for example, Patent Literature 2 discloses a technique of preventing power from being wastefully consumed as heat due to the same voltage supplied to anodes, in a color-sequential LED driving circuit in which R, G and B LEDs are sequentially turned on. The color-sequential LED driving circuit disclosed in Patent Literature 2 solves the problem of heat generation by (i) providing, between a power source circuit and an anode of an LED, a circuit which allows output of an anode voltage suitable for an LED to be turned on and (ii) switching the circuit depending on which LED is to be turned on.